This invention relates to the field of test instrumentation and more particularly to an instrument and method for studying supersonic compressible gas flow by the hydraulic analogy technique. In still greater particularity the invention relates to an instrument for simulating the thermodynamic property specific heat ratio of a gas for the proper application of the hydraulic analogy technique.
It is well known by those skilled in compressible and hydraulic flow analysis that an analogy exists between gas and hydraulic flow systems. This analogy exists due to the similarity in the equations that describe the two dimensional flow characteristics of the two systems, i.e., continuity, momentum and pressure equations, when the Mach number of the compressible flow is equal to the Floude number of the hydraulic flow. It has been desirable to take advantage of this similarity to study compressible flow by using an hydraulic system to study hydraulic flow on a model and analogize the hydraulic results to the compressible system.
The analogy allows the study of compressible flow systems such as airfoil or turbine blade characteristics without the need to construct and operate large scale wind tunnels. Small scale model tests can be performed, drastically reducing test system complexity and cost. The hydraulic analogy tests are presently carried out using a shallow, single layer of a liquid medium, normally water. The liquid is caused to flow past a stationary model of the device under study, or the model is moved through the stationary liquid. Measurements of the depth of the liquid are taken at points near the model where data is desired. The measured depths are analogous to the temperature, density and pressure that would exist in the compressible medium at these same locations. It is known that the hydraulic analogy assumes that the thermodynamic property known as specific heat ratio of the gas in the system under study is 2. The specific heat ratio of most gases however is in the range 1.2 to 1.5. At room temperature, the specific heat ratio of the most common gas, air, is 1.4. When studying a gas system having a specific heat ratio other than 2, considerable error is introduced into the hydraulic analogue because the single layer water systems can not simulate specific heat ratios other than 2 with any degree of accuracy. Heretofore, attempts to simulate specific heat ratios other than 2 have centered on using flow channels whose cross-sections are other than rectangular. For example, V-shape channels have been found to give the analogue a specific heat ratio of 1.5, but they are difficult in which to physically perform experimented work.
It can be appreciated that to protect the validity of the hydraulic analogue it is desirable to be able to simulate the property specific heat ratio of the gas in the system under study.